Replacement Components

Many components used in the Sinclair hardware, peripherals, etc. are no longer manufactured. However for most of these components there are modern alternatives. Much research was conducted by DEATH and others on the World of Spectrum forums

Spectrum +3

The transistors in the +3 PSU are H1061s. Possible alternatives include the BD243C and TIP41C

Integrated Circuits

RAM

MM5920N (national Semiconductors) chips are equivalent to 4116 RAM chips
AM9016EDC (AMD) chips are the same as 4116 chips.
IMS2600P-12 (Inmos) chips work in any upper RAM position (IC15-22)
41256 chips work in place of 4164 upper RAM chips
V53C256P80L chips work in any upper RAM position (IC15-22)
MN41257-12 chips work in any upper RAM position (IC15-22)

Logic

74LSxx series chips can be replaced with 74HCTxx series chips

CPU

Various makes of Z80 processor are used on Spectrums.
Zilog Z80 CPU
NEC D780C